 A part of the circuit consists of three capacitors, C1, C2, C3 as shown below. We know the charge stored on the capacitor C1, Q1 that is 75 micro coulombs. Charge stored on the capacitor C2 is 30 micro coulombs and the potential drop across C2 is 9 volts. So we know that the potential drop across C2 is 9 volts. We need to figure out the capacitance of the capacitor C3. All right, now in order to figure out capacitance, we can go back to what the relation of capacitance, charge and voltage was, right? So that was C3, in this case C3, this will be equal to Q3 divided by V3, right? We can already figure out what is V3 because the potential drop across C3 will be the same as the potential drop across C2. They are connected in parallel and we know this is 9 volts. So V3 already, we know that this is 9 volts. All we need to do is figure out Q3 and in order to figure out Q3, we can take a look at the setup. So, C1 is connected to C2 and C3. All the charge that will be accumulated on C2 and C3, it should be related to the charge on C1 in some way or the other, right? So let's take a look at, let's take a look at this figure right here. Let's assume that charge Q3 is deposited on the left plate of the capacitor C1. So that means minus Q1 will be deposited on the right plate and let's say Q2 is deposited on C2 and Q3 on C3. Now if we take a look at this rectangle right here, if we take a look at this rectangle, all the charges inside this rectangle, they are not really connected to any part of the circuit or like the plates inside this rectangle, they are not really connected to any part of the circuit. So that means it is an isolated system. By that, I mean the addition of all of the charges inside this rectangle should be equal to zero. So if we write that, this will be Q2, Q2 plus Q3, Q2 plus Q3 minus Q1, minus, minus Q1. This comes out to be equal to, this comes out to be equal to zero. And this will give us the relation of Q2 plus Q3, Q2 plus Q3 is equal to Q1. This is equal to, this is equal to Q1. We know what Q1 is, we know what Q2 is, we know Q2. So let's figure out Q3, Q3 will come out to be equal to Q1 that is 75, 75 minus 30 and this is 45, 45 micro coulombs. So then if we, this is micro coulombs, now if we replace 45 in this relation, this will give us 45 divided by 9 and this is 5, 5 micro faraday, 5 micro faraday.